[Asterisk-Users] POTS failover relays (was Vonage, PSTN, 911, and hardware question)

[Rajeev Sharma]

OK... so what you're saying is that I put a diode across the power supply input legs for the DPDT 
relay, right?

(sorry, i'm not the best person at electronics...)

Greg Hill wrote:
> On Sun, 10 Oct 2004, Rajeev Sharma wrote:
> 
> 
>>Yeah, thanks, I was thinking of doing something similar to that.
>>Actually, I was gonna spice a cable in my computer's power supply and
>>use that. Why? Because if it's on a UPS, then the switch will throw at
>>the same time as the computer looses off. I dunno, I might not even use
>>a UPS, just a surge protector, but I'll see. Thanks for the idea.
> 
> 
> I was going to suggest this, but didn't because it's (slightly) more
> involved. Due to the physics of a relay (that it's constructed with a coil
> of wire), there is some inherent inductance. When you try to interrupt the
> current to an inductor, the voltage across it spikes. This is considered a
> Bad Thing in sensitive electronics like a computer. This isn't really an
> issue with the wall-wart power adapter, because there isn't likely to be
> anything terribly sensitive in there (and they're cheap to replace in the
> event of failure).
> 
> In a sensitive computer environment, you should include a reverse-biased
> diode in parallel with the relay's coil.  This diode, because it's
> reverse-biased in normal circuit operation, won't conduct any current. But
> when power is lost, the voltaged induced by the inductor will forward bias
> the diode, and the voltage spike will be clamped by the diode rather than
> going out into other components via the power bus. Most any common diode
> will work fine (1N4148 small-signal diode, 1N4001/2/3/4 rectifier diode,
> etc) for this purpose.
> 
> Greg
> 
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